This invention relates to thermalimagers which use liquid crystals as the sensing elements.
It is known to use liquid crystal cells for thermal imaging. The crystals are contained by transparent plates which form the walls of the cell. In use, the liquid crystal is held at its cholesteric-isotropic phase transition temperature. At this temperature the crystal has a large temperature coefficient of optical rotatory power. When infra-red radiation falls on the crystal held at this temperature, it alters the crystal temperature locally, producing changes in phase within the crystal, ie from cholesteric to isotropic. These changes can be measured using a scanning polarimeter which gives an output electronic signal which is proportional to the incident infra-red radiation. However, the sensitivity of the liquid crystal cell is reduced due to the walls of the cell absorbing some of the incident radiation and locally heating the crystal. Problems with the flow of the liquid crystal may also occur due to the localised changes in phase caused by this local heating. Impurities on the internal walls of the cell may act as centres of nucleation, ie points onto which the crystals may attach themselves and produce undesirable molecular orientation effects.
Free-standing chiral smectic C liquid crystal films, having thicknesses between 10 to 100 molecules (500-5,000 .ANG.), are known to exhibit a strong temperature dependence in which the pitch of the helix alters with temperature. The term "free-standing" is applied to a liquid crystal film and is used to define those liquid crystal materials which have sufficient surface tension to form a film without the necessity of containment plates.